Fast playback has to do with frame rates.
Most content is recorded at 24 frames per second: Cinema Format.
In Europe, this is accelerated to 25 frames per second, because our TVs work on this. You can hardly hear them unless you compare them to each other.
However, in the USA this should go to 30fps, which is too many to speed up, and there they make a 3:2 pullout. This means you make 5 out of 4 frames, by interlocking 2 of those 5 frames. With each other. Interlacing is out of date, so this is blended/blurred: showing two frames at the same time by blending them together.
So in Europe: 4% faster playback, or in the USA: 40% blurred image.
However, if you don’t and leave it at 24fps, you can hope your TV will handle this well.
Televisions that cannot run this in Europe, every single second are doubling. It comes across stumbling. In the US this happens 6 times per second, which is much worse.
Now you have to ask yourself what HBO wants: users with a good image, but also users with a stuttering image. Or users where it always goes well, but is less good. Perhaps the last option is better for them: fewer complaints.
In my opinion, they would have both variants on the server and let the user choose, but that of course requires more work for HBO, and storage space.
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